For air-conditioning equipment for a large building such as an office building and an apartment house, a so-called multiple air conditioning system has been used in which a refrigerant is supplied from one outdoor unit to a plurality of indoor units installed in the building. In the large-scale system of this type, the outdoor unit is sometimes provided in plural numbers.
In the multiple air conditioning system, the required air cooling capacity or heating capacity differs depending on the number of operating indoor units. Therefore, to meet this condition, a plurality of compressors are mounted in the outdoor unit.
The case where two compressors are mounted is explained. Usually, as a first compressor, a variable-speed compressor in which the rotational speed is variable due to inverter control is used, and as a second compressor, a constant-speed compressor in which the rotational speed is constant is used.
Up to a predetermined capacity, the second compressor is not operated, and only the first compressor is operated with the rotational speed thereof being controlled by an inverter. In contrast, when a capacity higher than a predetermined value is required, the second compressor of constant-speed type is operated together with the first compressor.
Thus, in both of the case where only the first compressor is operated and the case where both of the first and second compressors are operated, the excess and deficiency of a refrigerator oil in the compressors must be prevented.
As one example of the preventive measures, in the invention described in Patent Document 1 (Japanese Patent Application Publication No. 2001-324230), the refrigerator oil storage part of the first compressor and the suction pipe of the second compressor are connected to each other via oil return piping having a pressure reducing means, and the refrigerator oil storage part of the second compressor and the suction pipe of the first compressor are connected to each other via oil return piping having a pressure reducing means.
According to the invention described in Patent Document 1, when both of the two compressors are operated, excess refrigerator oil is returned from the first compressor to the second compressor or from the second compressor to the first compressor, and when only one compressor is operated, excess refrigerator oil is returned to the own machine, so that a proper amount of refrigerator oil is held in the compressors.
The oil return piping is provided with a capillary tube serving as the pressure reducing means. Therefore, when the compressor is stopped, an equal pressure is not always provided on the discharge side and suction side thereof.
This presents a problem especially when the first compressor (inverter compressor) is restarted. Specifically, in the case of the inverter compressor, if the pressure difference between the discharge side and the suction side is large at the restart time, an excessive load is applied to a motor. Therefore, a motor protecting circuit is activated, and the starting may fail.